Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, different access networks may coexist in parallel. Typically, in relation to mobile communication systems, an access network is represented by a radio access network (RAN) which is based on a certain radio access technology (RAT). While “radio” is a typical medium for mobile communication, other media are intended to be also covered by the principles taught herein. For example, Infrared or Bluetooth® or other media and/or wavelengths of radio are possible to represent the access network. As there has to be a (downward) compatibility between newly developed and pre-existing access networks and/or access networks technologies, terminals often have a capability to communicate based on one or more access networks technologies. Also, when a new access network is developed and launched, the network is not immediately available in the entire country of deployment, but its coverage may be limited to certain areas and be successively expanded over time.
The present invention will herein below be explained with reference to LTE as one example of a access network or radio access technology (LTE is also known as fourth generation 4G mobile communication) and its successor or improvement which is currently being developed and referred to as 5G (fifth generation mobile communication) as a further access network or radio access technology. In particular, 5G radio access technology will be referred to as a first access network, while LTE or 4G will be referred to as a second access network. Though, principles set out herein below are applicable to other scenarios of first and second access networks, too. Typically, a mobile communication network consists of an access network establishing the physical transport of data (payload (user) data and control data) and a core network establishing the control functionality for the entire network and the interoperability of the network with other networks, e.g. via gateways. References to specific network entities or nodes and their names are intended as mere example only. Other network node names may apply in different scenarios while still accomplishing the same functionality. Also, the same functionality may be moved to another network entity. Therefore, the principles as taught herein below are not to be understood as being limited to the specific scenario referred to for explanation purposes.
For example, the Evolved Packet System (EPS) is the successor of General Packet Radio System (GPRS). It provides a new radio interface and new packet core network functions for broadband wireless data access. Such EPS core network functions are the Mobility Management Entity (MME), Packet Data Network Gateway (PDN-GW, P-GW) and Serving Gateway (S-GW).
FIG. 1 illustrates the Evolved Packet Core architecture as introduced and defined by 3GPP TS 23.401 v13.0.0.
The entities involved and interfaces there between are defined in that document and reference is made thereto for further details. Acronyms used in the Figure are listed at the end of this specification.
A common packet domain Core Network is used for both Radio Access Networks (RAN), the Global System for Mobile Communication (GSM) Enhanced Radio Access Network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN). This common Core Network provides GPRS services.
E-UTRAN, the evolved UTRAN represents the nowadays known 4G network. Its successor referred to as 5G network is under development.
It is envisioned that such 5G system will provide new mobile low-latency and ultra-reliable services, and some services like Vehicle-To-X (V2X) will be more efficiently provided by 5G system.
A reference to 5G architecture that is envisioned is depicted in FIG. 2.
The entities involved and interfaces there between are for example known from such document. Acronyms used in the Figure are listed at the end of this specification.
In brief, a terminal such as a 5G NT (network terminal or user equipment) is provided with an internet protocol IP user network interface (IP UNI) and an Ethernet user network interface (ETH UNI) and may communicate via a Uu* interface with an Access Point (AP) in the mobile access network. The entire network has a mobile access part and a networks service part and an application part. Within each of those parts, there exists a control plane and a user (data) plane. The AP is located in both planes.
During the early days of 5G deployment, it is expected that 5G coverage area is not nationwide. It is therefore desirable that a solution is developed to allow 5G devices to camp in other radio access technologies (e.g. LTE) that are widely available so that a terminal or user equipment UE does not lose the connection to the network immediately after losing 5G coverage.
FIG. 3 shows such an example scenario in simplified manner. A terminal 1, such as a user equipment UE, e.g. exemplified by a so-called smartphone or another portable communication device, may move due to its mobility from a position A to a position B. In position A, it experiences the coverage of a LTE (4G) network as a first access network as well as of a 5G network as a second access network. The coverage of a respective network is graphically illustrated by a respective hatching. The 5G network is represented by an access point AP denoted by 2. The 4G network is represented in this example by 3 eNB's (or three 4G access points) denoted by 3a, 3b, 3c, respectively. The 4G network has a greater coverage as the 5G network. The coverage of both networks overlaps at least partly as illustrated and denoted by the arrow labeled “4+5G”.
As shown, when moving from A to B, the terminal leaves the 4+5G coverage and enters the 4G only coverage. To the contrary, when moving from B to A, the terminal leaves the 4G only coverage and enters the 4+5G coverage.
Traditional inter RAT handover (HO) was designed for macro with bigger coverage area. Thus, frequent mobility with switching forth and back from one RAT to another RAT (ping-pong) was not expected. Traditional inter RAT HO starts from the premise that the border of each RAT is not widely spread over the coverage.
However, 5G cells are using high frequency spectrum (cmWave and mmWave), and thus the cell size will be small. Also considering that 5G is a throughput booster, the 5G and other RAT border will be much wider and frequent RAT changes are expected.
A possible (conceptual) inter RAT architecture following traditional concepts is depicted in FIG. 4.
The entities involved and interfaces there between are for example known from such document. Acronyms used in the Figure are listed at the end of this specification.
In brief, a terminal such as a 5G/LTE UE (network terminal or user equipment) may communicate with respective access points of a LTE (4G) network and a 5G network. The user plane and control plane of the 4G network is structured in the known way, while the user plane and control plane of the 5G network is structured as envisioned. Several interfaces (G1, Sha*, G2, G3) are provided between several entities of the user plane and the control plane of both networks to coordinate the parallel connection of the terminal to both networks. The physical transport of data is however controlled and conducted by the respective network to which the data is sent by the terminal.
Such architecture has the benefits that a dedicated core for each access is provided, and that interworking can be enabled with little or no changes to legacy nodes.
Nevertheless, such architecture in addition shows the following drawbacks:                architecture requires many new interfaces in order to support interworking thus increases complexity and latency of inter RAT HO,        architecture creates excessive signaling; traditional (inter RAT) HO was designed for interworking between macro cells with bigger coverage area; with 5G small cells in the higher end of the spectrum (cmWave, mmWave), this paradigm might change: frequent RAT changes may happen (it depends on how dense 5G is) thus making this architecture not suitable/agile,        statistics from field shows the issues with inter-RAT HO in general; besides that, inter RAT due to Circuit Switched Fallback (CSFB) also shows high increase of signaling, and        service interruption introduced by the frequent RAT changes may impact 5G services negatively.        
Furthermore, it is foreseen that Mobile Network Operators (MNO) may demand following characteristics:                new system should abide by the rules of the legacy networks (i.e. should not require software upgrade of existing deployed network elements),        existing Core Network (CN) elements can be upgraded to support both, 4G and 5G, at the same time, and        operators will have the possibility to upgrade CN elements selectively e.g. some Mobility Management Entities (MME) are upgraded to become control plane Mobile Gateway (cMGW) that can serve both 4G and 5G.        
Hence, in order to be able to fulfill varied requirements and at the same time to provide mobility robustness, the problem arises that a solution to improve dual connectivity ability was needed that does not significantly impact legacy networks and at the same time offers the required mobility robustness and enables fast switching.
Dual connectivity (DC) is defined in LTE with the primary objective to increase the throughput capacity per UE. The architecture of DC in LTE is as below.
FIGS. 5, 6 and 7 illustrate dual connectivity as defined in LTE.
In particular, FIG. 5 illustrates a control plane connectivity of evolved NodeBs (eNB) involved in dual connectivity, while FIGS. 6 and 7 illustrate user plane connectivity of eNBs involved in [LTE] dual connectivity (DC).
The entities involved and interfaces there between are defined in that document and reference is made thereto for further details. Acronyms used in the Figure are listed at the end of this specification.
In LTE DC, Radio Resource Control (RRC) signaling is provided only by Master eNB (MeNB) to UE, and MeNB is responsible to setup and to tear down DC, even though Secondary eNB (SeNB) can propose to tear down DC for local reason. Which bearer is to be served by SeNB is also decided by MeNB.
Combined attach procedure is supported for Packet Switched (PS) and Circuit Switched (CS) services (i.e. EPS attach and International Mobile Subscriber Identity (IMSI) attach; refer TS 23.272 version 12.4.0), however this is not supported for PS services from two different RATs.
Furthermore, currently for CS fallback to work, EPS services are suspended, and the UE falls back to 2G/3G RAT. Thus, the assumption is that there is only single connectivity and the combined registration procedure exists mainly for the CS core to obtain UE context and also to have the ability to reach the UE in case of Mobile Terminated (MT) calls.
Idle State Reduction (ISR) is a mechanism for a UE to be registered (attached) in two RATs at the same time when in idle mode but not in connected mode. The purpose is that a UE can recamp to different RATs without necessarily triggering a tracking area update (TAU).
Dual connectivity (DC) as defined in LTE can not solve the above problems and satisfy the mentioned requirements.
Hence, there is a need to provide for improvements in dual connectivity for different access networks.